Reduce image size

As described in #4 (closed)

This doesn't change semantics, but optimizes layers.

It reduced the compressed image size on my local registry from 1388 MB to 894.01 MB.

@bgamari

---

On gitlab registry:

$ docker manifest inspect --insecure registry.gitlab.haskell.org/maerwald/ci-images/x86_64-linux-alpine3_12:26975eeb899bdad04e7df8156d9ce0aea7c9986c | jq '[.la
yers[].size] | add'
1028462892
$ docker manifest inspect --insecure registry.gitlab.haskell.org/ghc/ci-images/x86_64-linux-alpine3_12:4d097a9a8a11adf5576a7434b04f265d0baf8e46 | jq '[.layers[
].size] | add'
1606661362

components/Cabal.dhall

@@ -26,13 +26,16 @@ let
   installFromSource: Text -> CF.Type =
     \(version: Text) ->
       let
-        path: Text = "/usr/local/bin/cabal"
+        path: Text = "/usr/local/bin"
+      let
+        storeDir: Text = "/opt/cabal/store"
       in
         CF.run "install cabal"
         [ "cabal update"
-        , "cabal install cabal-install==${version}"
-        , "mv $HOME/.cabal/bin/cabal ${path}"
+        , "mkdir -p ${storeDir}"
+        , "cabal install --store-dir=${storeDir}/store --enable-static --install-method=copy --installdir=${path} cabal-install==${version}"
         , "${path} --version"
+        , "rm -rf ${storeDir}"
         ]
       # setEnv path